agreeing about the use of the Vaporetto to see and admire the Grand Canal buildings and atmosphere.
When planning the time, consider that most workers in Venice don't live there. So they commute to and from work, so there are for sure rush hours at those times where the workers need to use the vaporetto services and add significantly to the crowding, sometimes very crowded. Add the workers going home to day trippers and cruise ship passengers all going back to the mainland in late afternoon and it is like a New York City subway at 5pm.
Boats going the other way can be mostly empty.
Workers arrive in the morning, but are a bit more spread out, and the day trippers and cruise ship passengers haven't arrived yet, or for the few which actually dock in Venice anymore - haven't finished breakfast.
So my experience is that there are golden hours. After dinner as the lights in the palaces come on, even later when it is dark, early morning when all is quiet, or a little later heading up the canal towards Ferrovia.
Late morning, early afternoon have less perfect photography light and more day trippers, but I have found if you get on at the right time and get the right seats, the vaporetto is perfect. I've never used a Venice water taxi.
I have a book with photos and stories of every building on the Grand Canal, better than any taxi driver, and Ricj Steves' book and tour is very good. Cheap too. Hard to beat free for a tour.